Microelectrodes for implantation into soft tissue, in particular tissue of the central nervous system (CNS), have a wide field of application (Brain Machine Interfaces. Implications for Science, Clinical Practice and Society. Schouenborg J, Garwicz M and Danielsen N, Eds. Progress in Brain Research, Elsevier Science Ltd. 2011, ISBN 13: 978-0-444-53815-4). In principle, all brain nuclei can be recorded from or stimulated by such electrodes and their functions monitored. Of particular interest are multichannel electrodes for brain nuclei stimulation. In multichannel electrode design, groups of electrodes or even individual electrodes can be addressed separately. This allows a user to select those electrodes whose stimulation produces a therapeutic effect that is improved in comparison with non-selective stimulation. Stimulation of the brain or spinal cord can be of particular value in situations when brain nuclei are degenerated or injured. A multichannel design may provide for efficient measurement of the effects of systemic or local drug administration or gene transfer on neurons of the brain and spinal cord. Monitoring brain activity through implanted electrodes can be used to control drug delivery locally or systemically or to control electrical stimulation of brain nuclei. Furthermore, multichannel electrodes may be used to lesion specific sites in tissue upon detection of abnormal electric activity by the same electrodes.
An implanted microelectrode should affect the adjacent tissue as little as possible. Since the brain, the spinal cord, and peripheral nerves exhibit considerable movements caused by body movements, heart beats, and respiration, it is important that an implanted electrode can follow the movements of the tissue with as little as possible displacement relative to target tissue. To this end an implanted electrode should be resiliently flexible. Different methods to implant flexible electrodes are known in the art. For example, ultrathin and flexible electrodes, which are difficult or impossible to implant as such, can be implanted after embedding them in a hard matrix, which provides necessary support during implantation. After implantation the matrix is dissolved by tissue fluid. A requirement for successful implantation is the use of a biocompatible matrix material.
A problem with microelectrodes known in the art is that most of their impedance is made up by the impedance at the electrode/body fluid boundary. When current is passed through a medical electrode into or out from tissue, the current density is not uniform over a microelectrode surface, being substantially higher at edges, tips and surface irregularities than elsewhere. High local current densities cause the temperature to rise locally, and may even result in hydrolysis of aqueous tissue fluid. Soft tissue adjacent to sites of high current density thus risks to be irreversibly damaged.
To record activity in single neurons, the portion of the electrode in electrical contact with tissue and/or tissue fluid should be as small as possible. Since electrode impedance depends, to a large extent, on the surface area of that portion, various means have been developed to enlarge the surface to reduced electrode impedance. Methods for enlarging the electrically conducting surface area of electrodes are known by the art; they include roughening the surface mechanically or chemically coating the electrodes or coating with nanofibers of an electrically conductive polymer such as poly(3,4-ethylenedioxythiophene; PEDOT or PEDT), platinum black or carbon nanotubes. A problem with such coats is that they easily detach from the electrode body and/or that they get covered and/or clogged upon implantation by biological material emanating from tissue and body fluid. Thereby, the surface area of the conductor is reduced resulting in an undesired change of impedance.